1. Field of the Invention
The present invention relates generally to a method and apparatus for reducing interference to electrical signals and, particularly, to a method and apparatus for increasing immunity of a signal to low frequency noise.
2. Background Art
Digital circuits typically utilize squarewave signals or transitions having binary levels. The binary levels or the transitions are used to represent data. Since the data represented by a signal changes over time, the timing of the signal is referenced to a timing standard, or clock. A clock typically provides a clock signal in the form of a regular squarewave of a fixed frequency and phase. Since the operation of a synchronous digital circuit may be synchronized by a clock signal, a synchronous digital circuit may malfunction if noise or other interference is present on the clock signal. Thus, it is necessary to keep a clock signal clear of such noise or interference.
Noise and interference is more likely to occur on a clock signal in clock distribution circuits having high output impedance or where the output impedance is mismatched from the line impedance. For example, if a long wire is used to distribute a clock signal, the long wire will act as a transmission line having a characteristic impedance. The clock signal will propagate down the transmission line until it reaches the end. If the end is unterminated, the clock signal will be reflected at the end and will propagate backward along the transmission line. Such a reflection can also occur at the original end of the transmission line. If reflections occur at both ends of the transmission line, a long series of reflections can continue to occur at alternating ends of the transmission line, causing substantial degradation of the clock signal. This degradation often appears as overshoot/undershoot and ringing. Overshoot occurs when a signal exceeds the positive limit of the acceptable voltage range for representing logical values. Similarly, undershoot occurs when a signal falls below the negative limit of the acceptable voltage range for representing logical values. Ringing is a decaying oscillation that is superimposed on a voltage representing a logical level following a transition.
FIGS. 1A, 1B, and 1C illustrate well-known techniques for eliminating overshoot/undershoot, ringing, and other degradation resulting from impedance mismatches. FIG. 1A illustrates one technique for eliminating impedance mismatch and associated problems. Buffer 101, which has an open collector or totem pole output, receives an input at node 105 and provides an output at node 106. Node 106 may be a long wire or PC board trace that acts as a transmission line. At the distant end of the signal line represented by node 106, a resistor 102 is coupled from the signal line to the positive supply voltage. A resistor 103 is coupled from the signal line to ground. The distant end of the signal line is also coupled to an input of Schmitt trigger buffer 104. The output of Schmitt trigger buffer 104 is coupled to node 107.
Since the input of Schmitt trigger buffer 104 has a relatively high input impedance, the signal propagating down the signal line at node 106 sees a termination impedance equal to the parallel combination of the resistor 102 and resistor 103. To provide proper impedance matching, values are chosen for resistors 102 and 103 such that the parallel combination of resistors 102 and 103 are equal to the characteristic impedance of the transmission line at node 106, while the ratio of resistors 102 and 103 allow buffer 101 to swing node 106 well into validity voltage ranges for logic levels zero and one.
FIG. 1B illustrates another technique for eliminating impedance mismatch and associated problems. Buffer 108, which has an open collector or totem pole output receives an input at node 114 and provides an output at node 115. Node 115 may be a long wire or PC board trace that acts as a transmission line. At the distant end of the signal line represented by node 115, a resistor 109 is coupled from the signal line to the positive supply voltage. A resistor 110 is coupled from the signal line to ground. The distant end of the signal line is also coupled to the first terminal of resistor 111. The second terminal of resistor 111 is coupled to the first terminal of capacitor 112 and to an input of Schmitt trigger buffer 113 at node 116. The output of Schmitt trigger buffer 113 is coupled to node 117.
As in FIG. 1B, a signal propagating down the transmission line at node 115 sees the parallel combination of resistors 109 and 110. Then, the signal is low-pass-filtered by the combination of resistor 111 and capacitor 112 to reduce the effects of high frequency noise. The filtered signal is then applied to Schmitt trigger buffer 113, which restores the desired square edges (fast transitions) to the signal.
FIG. 1C illustrates yet another technique for eliminating impedance mismatch and associated problems. Line driver 118, which has a high current output (such as that of a 74AC244 line driver), receives an input at node 122 and provides an output at node 123. Node 123 may be a long wire or PC board trace that acts as a transmission line. At the distant end of the signal line represented by node 123, the signal line at node 123 is coupled to the first terminal of capacitor 120. The second terminal of capacitor 120 is coupled to node 124 and to the first terminal of resistor 121. The second terminal of resistor 121 is coupled to ground. The distant end of the signal line is also coupled to an input of Schmitt trigger buffer 119. The output of Schmitt trigger buffer 119 is coupled to node 125.
The series combination of capacitor 120 and resistor 121 provides an AC termination to the distant end of the signal line at node 123. Line driver 118 provides a low impedance output to increase noise immunity. Schmitt trigger buffer 119 helps clean up distortion of the signal from line driver 118 that might occur as the signal propagates down the transmission line at node 123.
Although the circuits of FIGS. 1A-1C help reduce the effects of noise and interference on a signal being transmitted, they do not provide a high voltage barrier to isolate parts of a system that operate at voltages that may differ greatly. Furthermore, the circuits of FIGS. 1A-1C do not provides immunity to low frequency interference for high frequency signals. In fact, the circuit of FIG. 1B includes a low pass filter to reduce high frequency noise, but provides no protection against low frequency noise specifically. Thus, a method and apparatus is needed to reduce low frequency interference while transmitting a high frequency signal across a high voltage isolation barrier.